1. Field of the Invention
The present invention generally relates to packaging for integrated circuits, and more particularly to a method of testing an interposer used to interconnect multi-chip modules.
2. Description of the Related Art
Integrated circuits are used for a wide variety of electronic applications, from simple devices such as wristwatches, to the most complex computer systems. A microelectronic integrated circuit (IC) chip can generally be thought of as a collection of logic cells with electrical interconnections between the cells, formed on a semiconductor substrate (e.g., silicon). An IC may include a very large number of cells and require complicated connections between the cells. A cell is a group of one or more circuit elements such as transistors, capacitors, resistors, inductors, and other basic circuit elements grouped to perform a logic function. Cell types include, for example, core cells, scan cells and input/output (I/O) cells. Each of the cells of an IC may have one or more pins, each of which in turn may be connected to one or more other pins of the IC by wires. The wires connecting the pins of the IC are also formed on a surface of the chip.
As the size of integrated circuits continues to shrink and pin densities grow, it has become increasingly more difficult to interconnect the chip to external circuitry or other chips. One packaging technique that allows for tighter interconnection of multiple chips is the multi-chip module (MCM). An MCM has two or more IC chips (dies) mounted on a single substrate or carrier that provides direct connections between the mounted chips as well as connections to external circuitry such as a printed circuit board (PCB). MCMs achieve higher performance resulting from reduced signal delays between chips, improved signal quality between chips, and a reduced number of external components.
For high performance computing applications, the use of high-function, high-cost MCM processors has made it desirable to attach these modules to the PCBs so that they can be replaced without having to desolder the part, i.e., as field replaceable units. The use of multiple MCMs in a single circuit assembly has also increased. FIG. 1 illustrates one example of a high performance, glass-ceramic MCM assembly 1. MCM assembly 1 has two replaceable MCMs one of which 2 is shown. MCM 2 includes microprocessor dies 3a, 3b mounted on a cordierite glass-ceramic carrier 4 having internal copper wiring (MCM 2 has four microprocessors but only two are visible in the side view of FIG. 1). Contact pins of the MCMs are soldered to I/O pads on the top surface of carrier 4 that are connected to the internal wiring network. An underfill material (organic capsulant) is dispensed along the edges of the dies and cured, and a heat spreader 5 is applied to the top of the dies using a thermally conductive adhesive. A lid 6 is laid over the heat spreaders and secured to carrier 4 with a silicone adhesive and cured to form a seal about MCM 2. A set screw may be inserted into a threaded hole in the top of lid 6 to adjust a spar that tightens the MCM and limits substrate flexure. A heat sink 7 is mounted on lid 6.
An interposer 8 is used to provide interconnections between MCM 2 and a card or PCB 9. Interposer 8 is an insulating layer having a multitude of embedded conductive elements 10 arranged in a land grid array (LGA), and aligned with respective metallized I/O pads on the MCMs and PCB 9. Conductive elements 10 are made from a compressible metal, and a spring-loaded actuation system 11 is employed to apply a normal retention force through MCM 2 along the peripheral seal against interposer 8 and PCB 9. PCB 9 can be further supported by a steel plate/stiffener 12. Interposers are provided for each MCM in the assembly. Chips in different MCMs can accordingly communicate along a circuit path from one MCM through its interposer to PCB 9 and thence to the interposer of the second MCM.
While the use of interposers greatly simplifies replacement of an MCM, the conductive elements 10 have been known to fail, causing an electrical open along a circuit path between chips on different modules. The interconnections between MCMs can be tested by system diagnostics integrated into the MCM chips. In the case of an interposer failure which causes an electrical open, the diagnostic can indicate a broken connection but cannot isolate the fault, i.e., it cannot determine which interposer along the circuit path has the malfunctioning element. Consequently, two or more interposers might have to be replaced even though only one is defective. It would, therefore, be desirable to devise an improved method of testing the integrity of interposers in a chip module assembly which could isolate a fault to a specific interposer. It would be further advantageous if the method could be implemented without requiring modifications to the interposers or excessive overhead to the MCM diagnostic logic.